Micro-electro-mechanical systems (MEMS) switches used in wireless communication devices such as mobile terminals require a gate voltage of about 100V to activate. A high voltage charge-pump includes a high voltage output that can provide an output voltage in a range of 80V to 150V. The output voltage is required to stay within a specified voltage range due to voltage tolerances of various devices connected to the high voltage output. For example, the output voltage should not exceed a specified maximum voltage of a device such as a transistor and a high voltage level shifter, yet the output voltage should exceed the MEMS threshold voltage needed to provide a low resistive radio frequency (RF) switch contact.
FIG. 1 is a block diagram of a prior art feedback loop controlled high voltage charge-pump 10 that is usable to maintain an output voltage within a specified voltage range. A charge-pump section 12 boosts a source voltage Vdd to a higher level. In operation, the output voltage is detected by a resistive divider load 14 that provides a divided voltage VDIV. The divided voltage VDIV is compared to a reference band gap voltage VBG. In response, an integrator 16 outputs an error signal VTUNE that controls a voltage controlled oscillator (VCO) 18. A signal FOSC output from the VCO 18 proportionally controls the output voltage level of the charge-pump 10. When the output voltage of the charge-pump 10 drops below a specified voltage range set by the reference band gap voltage VBG, the frequency of the signal FOSC is increased by the VCO 18 to raise the output voltage of the charge-pump 10 such that the output voltage rises back within the set voltage range. Alternately, when the output voltage of the charge-pump 10 rises above the specified voltage range, the frequency of the signal FOSC is decreased by the VCO 18 such that the output voltage of the charge-pump 10 falls back within the set voltage range.
FIG. 2 depicts a circuit diagram of a prior art Dickson charge-pump 20 that is usable as the charge-pump section 12. The Dickson charge-pump 20 comprises N stages made up of capacitors C1 through CN and diodes D1 through DN+1. For example, a first stage is made up of the diode D1 having an anode coupled to the input voltage Vdd, and a cathode coupled to an anode of the diode D2. The capacitor C1 has a first end coupled to both the cathode of the diode D1 and the anode of the diode D2. The capacitor C1 has a second end that is driven by a first clock signal Ø. A second stage is made up of the diode D2 having a cathode coupled to an anode of the diode D3. The capacitor C2 has a first end coupled to both the cathode of the diode D2 and the anode of the diode D3. The capacitor C2 has a second end that is driven by a second clock signal Ø, which is inverted with respect to the first clock signal Ø. For subsequent stages, the odd numbered capacitors such as the capacitor C3 are driven by the first clock signal Ø, while the even numbered capacitors such as the capacitor C4 are driven by the second clock signal Ø. A first end of a filter capacitor COUT is coupled to the cathode of the diode DN+1. A second end of the filter capacitor COUT is coupled to a fixed voltage node such as ground. A load represented by a resistor RLOAD is coupled in parallel with the filter capacitor COUT.
FIG. 3 is a generalized block diagram of a prior art feedback loop controlled high voltage charge-pump 22. In this case, a detector 24 can be the resistive divider load 14 (FIG. 1). The detector 24 samples the output voltage Voutprovided by the charge pump 22 and outputs a detector voltage VDET that is received by an integrator loop filter 26. A band gap reference voltage VBG input into the integrator loop filter 26 outputs an error signal VERR that is proportional to the difference between the detector voltage VDET and the band gap reference voltage VBG. A VCO 28 receives the error signal VVERR, and in response outputs a variable frequency signal FOSC that is received by drivers 30 that output at least one clock signal CK that controls the level of the output voltage VOUT. In operation, the at least one clock signal CK increases in frequency when the output voltage VOUT drops below a desired level that is set by the band gap reference voltage VBG. A main charge-pump section 32 may be made up of the prior art Dickson charge-pump 20 (FIG. 2). The main charge pump section 32 boosts an input voltage Vdd to the voltage VOUT.
In an application wherein the prior art feedback loop controlled high voltage charge-pump 22 is used to activate a MEMS switch, a resistive load is practically nonexistent, drawing only about 100 nA depending on MEMS switching frequency. Therefore, any significant current drawn from the output of the main charge-pump section 32 is drawn by the detector 24. Even when providing a resistance of 50 MΩ for the detector 24, the amount of current drawn from the output of the main charge pump section 32 is on the order of 2 μA. Accounting for an efficiency of around 50%, a current drawn from a 2.7V source for Vdd will amount to about 150 uA, which is a significant energy drain for a battery operated device such as a mobile terminal.
Increasing the resistance of the detector 24 above 50 MΩ is not a solution to this current drain problem, because a 50 MΩ resistor takes up around 0.2 mm2, which is a relatively large integrated circuit (IC) die area. Also problematic is a relatively large parasitic capacitance that is associated with such a large resistor. The problems brought about by the relatively large capacitance are that the relatively large capacitance may limit the loop bandwidth and stability of the feedback loop controlled high voltage charge-pump 22. Increasing the complexity (i.e., the order) of the feedback may reduce the problems of loop bandwidth and stability, but increasing the complexity of the feedback would not improve the current drain problem or the problem of increased die size. Thus, there is a need for a circuit that reduces the relatively high current drain, while also reducing the amount of die area taken up by detector components used in the feedback loop of a high voltage charge-pump.